1. Field of the Invention
The present invention relates generally to an FM communication device. More specifically, the invention relates to an FM communication device which can avoid interference which tends to be caused by obstruction by signals of substantially the same channel, for example, and is suitable for use in a circumstance where a plurality of FM communication devices of substantially the same channel are used in a relatively narrow area, such as cord-less telephone devices.
2. Description of the Background Art
In a communication systems, such as cord-less telephone apparatus, it tends to occur that the service is obstructed during communication by an FM signal transmitted from other telephone apparatus of substantially the same channel as the communicating channel. This tends to occur frequently in case of the cord-less telephone systems, since a plurality of handy telephone apparatus commonly use a plurality of communication channels. In practical use, the user has to search for an open channel for communication. Therefore, when more than one telephone apparatus accesses the same channel, mutual interference occurs to cause obstruction of communication to each other. In order to avoid this, an obstruction detector circuit is installed in the FM communication devices currently in the market. The obstruction detector circuit is generally designed to detect obstruction of a communication for switching communication channel.
Conventionally, such an obstruction detector circuit is designed to AM-detect the intermediate frequency signal through the AM detection circuit. The AM detection circuit is so designed as to detect an AM-modulated intermediate frequency signal which is generated by an AM-modulation of the FM signal by another FM signal having a frequency band close to that of the former. Since the level of the output of the AM detection circuit is variable depending upon the level of the AM-modulated signal level, judgement can be made that interference occurs, when the output level of the AM detection circuit is higher than a predetermined reference level.
In case of the FM signal interference between cord-less telephone apparatus utilizing the same channel, a frequency difference is substantially small. Consequently, the AM-modulated signal level tends to be substantially low. Therefore, in order to detect obstruction between the same channel FM signals, the obstruction detector circuit must be provided with a substantially high sensibility for the AM-modulated signal. This can be provided by providing automatic gain control (AGC) circuit in the obstruction detector circuit. As set forth above, since the AM-modulated signal level is substantially low, the AGC circuit in the obstruction detector circuit has to have a substantially high gain.
For this reason, the conventional FM communication devices are provided with two high-gain intermediate frequency amplifier stages respectively for demodulation of the FM signal and for detecting obstruction. This limits the size of the FM communication device to make it compact, light-weight and inexpensive. Furthermore, due to presence of two high-gain intermediate frequency amplifiers, power consumption of the FM communication device was substantial.